"calculated according to the construction area" sounds like a good way to say it to me. I don't think anything else you could use would be better or more clear. Are the "property management fee and other fees" part of "the relevant fees set out in this agreement"? If So, I would say "The relevant fees set out out in this agreement (including the property management fee)". I do not think "and other fees" is necessary at all. If you do not want to use the parentheses (), or remove "and other fees," you still need to place a comma after "other fees,"

"determined by" is an easier way to say essentially the same thing. "found by measuring" or "proportional to" these are all different ways of communicating that meaning.